Molding sand comprising biodegradable polymeric binder

ABSTRACT

Molding sand containing (i) 100 parts by weight of refractory quartz or olivine matrix and (ii) 1 to 5 parts by weight of organic binder. The organic binder contains a mixture of (a) 40-50% w/w aqueous solution of sodium poly(acrylate) in the amount of 30-70% by weight and (b) 40-50% w/w aqueous solution of dextrin in the amount of 30-70% by weight.

CROSS-REFERENCE TO RELATED APPLICATIONS

No foreign or domestic priority claims are being made at the presenttime. Inquiries from the public to applicants or assignees concerningthis document should be directed to: Matthias Scholl P.C., Attn.: Dr.Matthias Scholl Esq., 14781 Memorial Drive, Suite 1319, Houston, Tex.77079.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to molding sand comprising biodegradable polymericbinder, for foundry use.

2. Description of the Related Art

Molding sand, also known as foundry sand, is sand that tends to packwell and hold its shape. It is used in the process of sand casting.

Sand casting, also known as sand molded casting, is a metal castingprocess characterized by using sand as the mold material. The term “sandcasting” can also refer to an object produced via the sand castingprocess. Sand castings are produced in specialized factories calledfoundries. Over 70% of all metal castings are produced via a sandcasting process.

Sand casting is relatively cheap and sufficiently refractory even forsteel foundry use. In addition to the sand, a suitable binder is mixedor occurs with the sand. The mixture is moistened, typically with water,but sometimes with other substances, to develop strength and plasticityof the clay and to make the aggregate suitable for molding. The sand istypically contained in a system of frames or mold boxes known as aflask. The mold cavities and gate system are created by compacting thesand around models, or patterns, or carved directly into the sand.

There are four main components for making a sand casting mold: basesand, a binder, additives, and a parting compound.

Binders are added to a base sand to bond the sand particles together(i.e. it is the “glue” that holds the mold together).

A mixture of clay and water is the most commonly used binder. There aretwo types of clay commonly used: bentonite and kaolinite, with theformer being the most common.

Oils, such as linseed oil, other vegetable oils and marine oils, used tobe used as a binder, however due to their increasing cost, they havebeen mostly phased out. The oil also required careful baking at 100 to200° C. (212 to 392° F.) to cure (if overheated the oil becomes brittle,wasting the mold).

Resin binders are natural or synthetic high melting point gums. The twocommon types used are urea formaldehyde (UF) and phenol formaldehyde(PF) resins. PF resins have a higher heat resistance than UF resins andcost less. There are also cold-set resins, which use a catalyst insteadof a heat to cure the binder. Resin binders are quite popular becausedifferent properties can be achieved by mixing with various additives.Other advantages include good collapsibility, low gassing, and theyleave a good surface finish on the casting. MDI (methylene diphenyldiisocyanate) is also a commonly used binder resin in the foundry coreprocess.

Sodium silicate [Na₂SiO₃ or (Na₂O)(SiO₂)] is a high strength binder usedwith silica molding sand. To cure the binder carbon dioxide gas is used.

The advantage to this binder is that it occurs at room temperature andquickly. The disadvantage is that its high strength leads to shakeoutdifficulties and possibly hot tears in the casting.

Although much progress in the area of molding sand binders has been madeover the years, much opportunity remains. Particularly, binders areneeded that have better biodegradability, are easier to cross-link, andare easier to regenerate, all while providing molds and cores that havehigh microstructural homogeneity.

SUMMARY OF THE INVENTION

In one aspect, this invention provides molding sand comprising based onweight of the molding sand: (i) 100 parts by weight of refractory quartzor olivine matrix and (ii) 1 to 5 parts by weight of an organic binder,the binder comprising a mixture of: (ii-a) an aqueous solution of sodiumpoly(acrylate), in an amount of 30-70% by weight with respect to theweight of the binder, and (ii-b) an aqueous solution of dextrin, in anamount of 30-70% by weight with respect to the weight of the binder.

When using the mixture of sodium poly(acrylate) and dextrin as a binder,it has been observed that the binder features much better adhesionproperties, allowing for a formulation of molding sand with much higherstrength compared to existing formulations. It is significant also thatthe binder more readily distributes in the binder-matrix compound.Moreover, the binder shows much higher biodegradability compared toexisting formulations.

The first component of the binder is sodium poly(acrylate) having polarproperties. It is very highly soluble in water and, as a salt of astrong base, it maintains the pH within a range of 8-9, which affectsthe basic reaction of the binder and does not prevent itsbiodegradability. It is used as a 40-50% aqueous solution. Sodiumpoly(acrylate) has good adhesive properties, viscosity and wettabilityof the matrix grains (relatively low wettability angle). Furthermore, anactive contribution of carboxylate groups (—COONa) present in sodiumpoly(acrylate) to the process of physical hardening of the molding sand,influences the achievement of highly cross-linked form. Molds and coreshaving higher strength are thus achieved in the hardening process.

The average weighted molar mass of sodium poly(acrylate) as used hereinis about: 4,000, 10,000, 56,000, 60,000, 94,000, 123,000, 132,000,156,000, 166,000, or 250,000 g/mol.

The second component is a modified biopolymer in the form of dextrin,which is well soluble in water (e.g., much better than carboxymethylstarch). Dextrin is used as a 40-50% aqueous solution. It has goodviscosity and adhesion in relation to the binder-matrix compound. Inaddition, dextrin is fully biodegradable.

The average weighted molar mass of dextrin as used herein is about:4,000, 6,000, or 8,000 g/mol.

In certain embodiments of the invention, a molding sand binder wasprepared

Molds or cores made of the above compound are subjected to cross-linkingreaction by microwaves or heat.

The sand of the invention is easy to break, and the water-solublebiopolymer contained in it causes the binder biodegradability, which isimportant for the storage of used sand and binder reactivity whilehardening, and allows cross-linking with a physical factor. Furthermore,the used sand not fully burnt, can be subjected to a refreshing process,which allows easy recycling thereof. The compounds are also easier toregenerate thermally or mechanically. Moreover, the molds and coresallow high microstructural homogeneity and, thus, the mechanical,usability and corrosion properties across the entire cast, and goodsurface quality is achieved. The casts are free from such defects as:pits, roughness, gas porosity or graphite deformation at the subsurfacelayer. The compound causes minimum hazards to the workers andenvironment.

Whenever the term “molding sand” is used herein, it encompasses “moldingsand”, i.e., sand that is used as the mold material to fill the castingflask, and “core sand,” i.e., sand that is used to make cores to beplaced into the mold to create the interior contours of the casting.

DETAILED DESCRIPTION OF THE INVENTION Examples

The following general procedures were applicable to all examples.

A 250 g sample of polymer binder was formulated by mixing 40-50% aqueoussolution of sodium poly(acrylate) with 40-50% aqueous solution ofdextrin in a weight ratio of 1:1.

The process of mixing polymer components of the binder was performedusing: (a) mechanical mixing with a mechanical mixer, PX-SR 90Dlaboratory mixer at 1000 rpm, mixing time 40 min., with the generalbinder preparation time of 60 min., (b) ultrasonic mixing using anultrasonic device, Polsonic, Sonic 3 model, frequency 40 kHz, ultrasonicexposure of 40 minutes, the total preparation time of the polymer binder60 minutes, or (c) hand mixing at an elevated temperature in a heatingjacket, water temperature in the heating jacket was 60-80° C., totalpreparation time of the binder of 60 minutes.

Molding compounds were prepared as follows. To the Ms-017A vane mixer,100 parts by weight of mineral matrix of quartz molding sand obtainedfrom Jaworzno Szczakowa mine were added (sand type: 1K-0,2/0,16/0,32,PN-85/H-11001). Then, the specified amount of binder of 1 to 3 parts byweight, were added and the parts were mixed for 3 minutes.

LUZ-1 WADAP vibration device was a part of the densification system. Thedevice was provided with a control module allowing for adjustment ofvibration time and amplitude. The vibration frequency was a constant 50Hz. The device was also provided with an execution module which enabledpreparation of up to nine slabs having an equal compaction degree.

Molds or cores made of the above compound are subjected to cross-linkingreaction by microwaves or heat.

Bending strength (for slabs) and compression strength (for rolls) weremeasured after hardening of the molding compounds. Measurements weremade periodically at specified intervals of 1 h, 2 h, 3 h, and 24 husing the LRu-2e device for testing molding sand strength according tonorm PN-83H-11073/EN. Strength values are averages of at least 6measurement results.

All experiments and measurements were conducted at 20° C. (±2° C.) andrelative air humidity of 45-50%.

Example 1

Molding sand comprising 100 parts by weight of quartz sand and 2.5 partsby weight of binder containing: 45% aqueous solution of sodiumpoly(acrylate) of 50% by weight and 40% aqueous solution of dextrin of50% by weight.

Sodium poly(acrylate) was purchased from BASF (catalog numberNJTS-50163-NVE, polymer in water, 45%-aqueous solution) and had thefollowing properties: average weighted molar mass Mw=4,000 g/mol, pH=9,and Brookfield viscosity=500 mPas. Dextrin was purchased fromSIGMA-ALDRICH® (ID 24858939, product number 31405 Fluka, dextrin frompotato starch) and had the following properties: average weighted molarmass Mw=4,000 g/mol, pH=8.5, and Brookfield viscosity=400 mPas.

A mold was made of the sand, which was placed in a heating system at150° C. for 120 minutes. After hardening, compound with compressionstrength after 24 hours storage=2 MPa, and bending strength after 24hours storage=1.3 MPa was obtained.

Example 2

Core sand comprising 100 parts by weight of olivine sand and 2.5 partsby weight of binder containing 40% aqueous solution of sodiumpoly(acrylate) of 50% by weight and 40% aqueous solution of dextrin of50% by weight.

Sodium poly(acrylate) was purchased from DWORY SA (polymer in water, 35%aqueous solution) and had the following properties: average weightedmolar mass Mw=250,000 g/mol, pH=8.5, and Brookfield viscosity=5,000mPas. Dextrin was purchased from SIGMA-ALDRICH® (ID 24858939, productnumber 31405 Fluka, dextrin from potato starch) and and had thefollowing properties: average weighted molar mass Mw=4,000 g/mol, pH=7,and Brookfield viscosity=400 mPas.

A core was made of the compound, which was then exposed to microwaveradiation from a source having a power of 800 W for 90 seconds. Afterhardening, compound with compression strength after 24 hours storage=2.5MPa, and bending strength after 24 hours storage=1.5 MPa was obtained.

Example 3

Core sand comprising 100 parts by weight of quartz sand and 3 parts byweight of binder containing 40% aqueous solution of sodiumpoly(acrylate) of 50% by weight and 40% aqueous solution of dextrin of50% by weight.

Sodium poly(acrylate) had the following properties: average weightedmolar mass Mw=4,000 g/mol, pH=8.5, and Brookfield viscosity=400 mPas.Dextrin was purchased from SIGMA-ALDRICH® (ID 24858939, product number31405 Fluka, dextrin from potato starch) and and had the followingproperties: average weighted molar mass Mw=4,000 g/mol, pH=7, andBrookfield viscosity=400 mPas.

A core was made of the compound, which was then exposed to microwaveradiation from a source having a power of 800 W for 90 seconds. Afterhardening, compound with compression strength after 24 hours storage=2.7MPa, and bending strength after 24 hours storage=1.5 MPa was obtained.

This invention is not to be limited to the specific embodimentsdisclosed herein and modifications for various applications and otherembodiments are intended to be included within the scope of the appendedclaims. While this invention has been described in connection withparticular examples thereof, the true scope of the invention should notbe so limited since other modifications will become apparent to theskilled practitioner upon a study of the drawings, specification, andfollowing claims.

The invention claimed is:
 1. Molding sand, comprising: (i) 100 parts byweight of refractory quartz or olivine matrix, and (ii) between 1 and 5parts by weight of an organic binder, wherein the organic bindercomprises a mixture of: (ii-a) 30-70% by weight with respect to theweight of the organic binder of a 40-50% w/w aqueous solution of sodiumpoly(acrylate), and (ii-b) 30-70% by weight with respect to the weightof the organic binder of a 40-50% w/w aqueous solution of dextrin. 2.The molding sand of claim 1, wherein the sodium poly(acrylate) has aweight average molecular weight Mw of 4,000-250,000 g/mol, pH of 7-9,and Brookfield viscosity of 400-5000 mPas.
 3. The molding sand of claim1, wherein the dextrin has a molar mass Mw of 4,000-8,000 g/mol, pH of6.5-7.5, and a Brookfield viscosity of 4000 mPas.